In cancer therapy, heavy particle radiotherapy using heavy ions, such as carbon ions, is featured by high biological effect on tumor, high dose concentration, and low-dose exposure to normal tissue surrounding a lesion. The known heavy particle radiotherapy includes a process of confirming a treatment region on the basis of prior diagnosis using a CT, and the like, a treatment planning process in which the beam diameter, the beam range, and the beam scan width are determined according to the treatment region, and a process of performing the irradiation according to the treatment plan. In a therapy apparatus used for the therapeutic irradiation, a quadrupole magnet is inserted at a beam transport line to modulate the diameter of a beam during beam transportation, and the irradiation position of the beam. Also, a ridge filter and a range shifter are provided, which enable the irradiation thickness and the irradiation position of the beam in an irradiation target to be changed. Optimum irradiation conditions about the beam diameter, the beam range, the beam scan width, and the like, are determined by performing beam simulation. In order to realize a beam transport system of heavy particles, a beam transport optical system, which is configured by a transport pipe evacuated to high vacuum, and various electromagnets which modulate the formation of the beam shape and the beam trajectory, needs to be designed so as to satisfy the beam specification required for using the beam. Further, in view of both machining accuracy and cost, it is desired that the weight of components configuring the beam transport system is small. For this reason, the inner diameter of the transport pipe is set to about several tens mm in order to reduce the weight of various electromagnets (Cancer Treatment in the 21st Century, Basics and Clinical Treatment of Heavy Particle Radiotherapy, edited by Hirohiko Tsujii, and published by Iryo Kagaku Sha, ISBN4-900770-83-3 C3047).
On the other hand, in cancer treatment, the diameter of the irradiation field needs to be set to about 210 mm in order to cope with the irradiation to the whole pelvic area, and the like. In order to satisfy this requirement, a conventional method for controlling beam irradiation (wobbler irradiation method) is described in Patent Literature 1 (1).
In the wobbler irradiation method, in order to shape an irradiation field of a beam with high accuracy, the size of the beam is increased by wobbler electromagnets which deflect the beam trajectory in a circular pattern, so that a uniform irradiation field is shaped by a metal collimator, and the like, so as to correspond to the cross sectional shape of cancer. However, when the metal collimator is adopted, much cost and time are consumed to manufacture the metal collimator. In addition, there is a problem that, when the broadened beam is shaped by the metal collimator, unintended radiation, such as a neutron beam, a gamma ray, and a proton beam, is generated to influence the accuracy of dose estimation.
To cope with this, an invention entitled “Charged Particle Beam Apparatus and Its Operation Method” is proposed in Patent Literature 1. With the invention described in Patent Literature 1, in the state where horizontal irradiation points and a required irradiation dose of a charged particle beam are determined beforehand by a calculation program software 131 on the basis of information on a lesion shape, and the like, and where the interval between the horizontal irradiation points is preferably set to about a half or less of the diameter of the charged particle beam expanded by a scattering body 300, a power supply apparatus 160 of electromagnets 220 and 221 used for setting the irradiation position is modulated by a control apparatus 132 so as to enable a uniform irradiation field to be shaped with a reduced loss of the charged particle beam (2).
The principle of the invention is based on the scanning method in which, as in a beam irradiation apparatus 1a shown in FIG. 4, a beam 2, taken out in the atmosphere after passing through a scattering body 9 provided at an end portion of a vacuum transport pipe 3, is modulated and irradiated onto an irradiation target 8 by being scanned transversely (in the horizontal direction (X-axis direction) and the vertical direction (Y-axis direction)) by using XY axis direction scanning electromagnets 6 and 7.
Further, in the invention described in Patent Literature 1, as shown in FIG. 4, a moderator (range shifter 11), which has adjustable thickness and which reduces the beam energy, is provided subsequently to the transverse direction scanning electromagnets 6 and 7, and the reaching distance (range) of the beam in the inside of the body is adjusted by making the beam 2 pass through the range shifter 11. Further, the beam 2 is dispersed by a ridge filter 10 in the traveling direction of the beam, so that a large irradiation field in the irradiation target 8 is obtained. However, the beam 2 is “dispersed” by passing through the range shifter 11, and hence the beam diameter is conventionally limited to about 10 mm according to (2).
FIG. 5 shows results of calculation of the change in the diameter of the beam in the irradiation target when the beam range is changed by changing the thickness of the range shifter in the beam irradiation apparatus shown in FIG. 4. The calculation was performed by using the beam optics calculation software “WinAgile” under the conditions that the beam energy was set to 235 MeV/u, and that the distance between the scattering body 9 and the body surface was set to 2 mm. FIG. 3 also shows results of calculation performed under the similar conditions.
In FIG. 5, the ordinate (D) represents the beam diameter (mm), and the abscissa (R) represents the beam range (mm). The frame left end indicated by arrow A corresponds to the surface of the irradiation target 8. The ordinate value of 0 corresponds to the design orbit of the beam. In FIG. 5, the beam diameter is set to about 2 mm. The abscissa (R) is set in a range of 350 mm from the abscissa value of 0 (the surface A of the irradiation target).
The calculation result “a” represents a beam behavior at the time when the thickness of the range shifter 11 is set to 30 mm. At this time, the maximum beam diameter at the maximum depth of the beam 2 is 3.828 mm. The calculation result “b” represents a beam behavior at the time when the thickness of the range shifter 11 is set to 20 mm. At this time, the maximum beam diameter at the maximum depth of the beam 2 is 3.666 mm. The calculation result “c” represents a beam behavior at the time when the thickness of the range shifter 11 is set to 10 mm. At this time, the maximum beam diameter at the maximum depth of the beam 2 is 3.3486 mm. The calculation result “d” represents a beam behavior at the time when the thickness of the range shifter 11 is set to 0 mm. At this time, the maximum beam diameter at the maximum depth of the beam 2 is 3.342 mm.
The beam range in the calculation result “a” is different from the beam range in the calculation result “d” by 30 mm, but the difference in the maximum beam diameter between the calculation result “a” and the calculation result “b” at this time is about 0.5 mm.
Further, an invention entitled “Particle Beam Irradiation Apparatus and Particle Beam Therapy Apparatus” described in Patent Literature 2 also discloses a beam irradiation apparatus which modulates the beam diameter. The particle beam irradiation apparatus according to the invention described in Patent Literature 2 is configured by including: a variable range shifter 4 which reduces the energy of a charged particle beam; a quadrupole magnet 6 which converges the divergence of the charged particle beam due to the scattering in the variable range shifter 4 by modulating the excitation amplitude of the electromagnet in correspondence with the energy of the charged particle beam reduced in the variable range shifter 4; and a scanning magnet 8 which changes the beam trajectory of the charged particle beam, and is configured such that an increase in the beam diameter of the charged particle beam due to the scattering in the range shifter can be reduced so that a charged particle beam having a small beam diameter can be supplied to enable spatially accurate irradiation to be performed to a body to be irradiated, and such that the range shifter can be arranged at a position separated from a patient, so as to eliminate an intimidating feeling due to the mechanical noize, and the like, of the range shifter.
In Patent Literature 2, the quadrupole magnet 6 is adopted to modulate, in the non-vacuum environment, the trajectory of the beam emitted from the transport pipe. However, since the quadrupole magnet 6 is arranged on the downstream side of the variable range shifter 4, the betatron function of the beam is destroyed at the time when the beam passes through the variable range shifter 4, and the emittance of the beam is increased. Therefore, in order to actually focus the beam, a distance of about 10 m needs to be provided between the variable range shifter 4 and the quadrupole magnet 6. The straight portion of the extraction port including the transport pipe and the extraction nozzle generally has a length of 10 m or less, and hence, even when the quadrupole magnet 6 is arranged on the downstream side of the variable range shifter 4, it is very difficult to plan the arrangement in which the diameter of the beam is maintained to be small until the beam reaches the inside of the irradiation target. Further, the beam diameter is changed by about 0.5 mm due to the thickness of the variable range shifter 4 as described above, and hence, in order to adjust the change in the beam diameter, it is necessary to highly accurately adjust the excitation pattern of the quadrupole magnet 6 for each of the variable range shifter 6 having different thicknesses. However, for the practical use of the technique disclosed in Patent Literature 2, a large amount of labor is required because it is necessary to manage the accuracy of the power supply of the quadrupole magnet 6 and to cope with the increase in the number of excitation patterns of the quadrupole magnet 6. Therefore, with the technique disclosed in Patent Literature 2, it is actually difficult to shape a beam having a diameter of several millimeters or less.
Until now, no technique has been known for shaping, in a target, a therapeutic beam having a diameter of about several millimeters or less without using a collimator. Further, no method has also been known for modulating the beam spot size in the irradiation target 8 with high accuracy of, for example, 0.1 mm or less, while securing a wide irradiation field.